Hellions are used in Terran vs Zerg and a little Terran vs Terran. They destroy Zerglings, Banelings, and Marines with ease, but have trouble against anything that can tank some damage. They also kill workers like a boss.

They are at least in my opinion the ideal unit for harassment, although yeah they suck at most other things. Really they just keep the other guys busy while I set up MMM and/or Vikings/ Siege Tanks. If you are really good at harassment, you can sometimes buy yourself the time to churn out a couple Thors for maximum